feat(openai-compat): honor OpenAI tool_choice on /v1 when tools passthrough is enabled

[cidrblock]

Context

PR for #73 adds opt-in tools passthrough on /v1/chat/completions but does not map OpenAI tool_choice (auto / none / required / specific function). Provider defaults apply.

Flagged during implementation of #73.

Proposal

When openai_compat tools mode is passthrough, forward or map tool_choice through the chat/AI SDK path so clients (e.g. Open WebUI) can force or suppress tool use.

References

• feat(openai-compat): opt-in tools support for /v1/chat/completions #73
• DR-032